Perturbation of cortical activity elicits regional and age-dependent effects on unconstrained reaching behavior: a pilot study.

Contributions from premotor and supplementary motor areas to reaching behavior in aging humans are not well understood. The objective of these experiments was to examine effects of perturbations to specific cortical areas on the control of unconstrained reaches against gravity by younger and older adults. Double-pulse transcranial magnetic stimulation (TMS) was applied to scalp locations targeting primary motor cortex (M1), dorsal premotor area (PMA), supplementary motor area (SMA), or dorsolateral prefrontal cortex (DLPFC). Stimulation was intended to perturb ongoing activity in the targeted cortical region before or after a visual cue to initiate moderately paced reaches to one of three vertical target locations. Regional effects were observed in movement amplitude both early and late in the reach. Perturbation of PMA increased reach distance before the time of peak velocity to a greater extent than all other regions. Reaches showed greater deviation from a straight-line path around the time of peak velocity and greater overall curvature with perturbation of PMA and M1 relative to SMA and DLPFC. The perturbation increased positional variability of the reach path at the time of peak velocity and the time elapsing after peak velocity. Although perturbations had stronger effects on reaches by younger subjects, this group exhibited less reach path variability at the time of peak velocity and required less time to adjust the movement trajectory thereafter. These findings support the role of PMA in visually guided reaching and suggest an age-related change in sensorimotor processing, possibly due to a loss of cortical inhibitory control.

What is the functional relevance of reorganization in primary motor cortex after spinal cord injury?

Motor output maps within primary motor cortex are widely distributed and modified by motor skill learning and neurological injury. Functions that these maps represent after spinal cord injury remain debatable. Moreover, the pattern of reorganization and whether it supports recovery of compromised motor function is not well understood. A deeper understanding of the pathophysiological mechanisms of motor dysfunction after spinal cord injury is necessary to develop and optimize repair strategies. There are three purposes for this review. The first is to synthesize available research on spontaneous reorganization with primary motor cortex following spinal cord injury. The second is to draw on existing evidence from the motor skill learning and brain injury literature to interpret the form and purpose of motor maps. The third purpose is to account for the existing research on intervention-induced reorganization of primary motor cortex following spinal cord injury. We conclude with directions for future work, emphasizing the need for multi-modal investigations that construct maps with both neuroimaging and non-invasive stimulation methods to derive a cohesive understanding of the effects of spinal cord injury on reorganization within primary motor cortex.

Spike-timing-dependent plasticity in lower-limb motoneurons after human spinal cord injury.

Recovery of lower-limb function after spinal cord injury (SCI) likely depends on transmission in the corticospinal pathway. Here, we examined whether paired corticospinal-motoneuronal stimulation (PCMS) changes transmission at spinal synapses of lower-limb motoneurons in humans with chronic incomplete SCI and aged-matched controls. We used 200 pairs of stimuli where corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the leg representation of the motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the tibialis anterior (TA) muscle 2 ms before antidromic potentials evoked in motoneurons by electrical stimulation of the common peroneal nerve (PCMS+) or when antidromic potentials arrived 15 or 28 ms before corticospinal volleys (PCMS-) on separate days. Motor evoked potentials (MEPs) elicited by TMS and electrical stimulation were measured in the TA muscle before and after each stimulation protocol. After PCMS+, the size of MEPs elicited by TMS and electrical stimulation increased for up to 30 min in control and SCI participants. Notably, this was accompanied by increases in TA electromyographic activity and ankle dorsiflexion force in both groups, suggesting that this plasticity has functional implications. After PCMS-, MEPs elicited by TMS and electrical stimulation were suppressed if afferent input from the common peroneal nerve reduced TA MEP size during paired stimulation in both groups. In conclusion, PCMS elicits spike-timing-dependent changes at spinal synapses of lower-limb motoneurons in humans and has potential to improve lower-limb motor output following SCI.NEW & NOTEWORTHY Approaches that aim to enhance corticospinal transmission to lower-limb muscles following spinal cord injury (SCI) are needed. We demonstrate that paired corticomotoneuronal stimulation (PCMS) can enhance plasticity at spinal synapses of lower-limb motoneurons in humans with and without SCI. We propose that PCMS has potential for improving motor output in leg muscles in individuals with damage to the corticospinal tract.

Validity of body-worn sensor acceleration metrics to index upper extremity function in hemiparetic stroke.

BACKGROUND AND PURPOSE: Accelerometers have been used to capture real-world use of the paretic upper extremity in people with stroke. It may be possible to characterize different aspects of the recorded acceleration to gain insight about movement capabilities during task-specific behavior. These measures may be of value for guiding rehabilitation. We undertook a study to identify the acceleration characteristics that have a stable association with upper extremity function and sensitivity to within-participant fluctuations in function over multiple sessions of task-specific training. METHODS: Twenty-seven adults 6 months or more poststroke with upper extremity paresis participated. Signals from wrist-worn accelerometers were sampled at 30 Hz during 7 sessions of task-specific training. Paretic upper extremity function was evaluated with the Action Research Arm Test. We used Spearman correlations to examine within-session associations between acceleration metrics and Action Research Arm Test performance. A mixed model was used to determine which metrics were sensitive to within-participant fluctuations in upper extremity function across the 7 training sessions. RESULTS: Upper extremity function correlated with bilateral acceleration variability and use ratio during 5 and 6 sessions, respectively. Time accelerating between 76% and 100% of peak acceleration correlated with function in 6 sessions. Variability of the paretic upper extremity acceleration and the ratio of acceleration variability between upper extremities were associated with function during all 7 sessions. Variability in both the acceleration of the paretic upper extremity, and acceleration of the paretic and nonparetic extremities combined were sensitive to within-participant fluctuations in function across training sessions. DISCUSSION AND CONCLUSIONS: Multiple features of the acceleration profile track with upper extremity function within and across sessions of task-specific training. It may be possible to monitor these features with accelerometers to index upper extremity function outside of clinical settings.Video Abstract available for more insights from the authors (see Supplemental Digital Content 1, http://links.lww.com/JNPT/A91).

Acceleration metrics are responsive to change in upper extremity function of stroke survivors.

OBJECTIVES: To (1) determine whether acceleration metrics derived from monitoring outside of treatment are responsive to change in upper extremity (UE) function; and secondarily to (2) compare metric values during task-specific training and while in the free-living environment, and (3) establish metric associations with an in-clinic measure of movement capabilities. DESIGN: Before-after observational study. SETTING: Inpatient hospital (primary purpose); outpatient hospital (secondary purpose). PARTICIPANTS: Individuals (n=8) with UE hemiparesis <30 days poststroke (primary purpose); individuals (n=27) with UE hemiparesis ≥6 months poststroke (secondary purpose). INTERVENTION: The inpatient sample was evaluated for UE movement capabilities and monitored with wrist-worn accelerometers for 22 hours outside of treatment before and after multiple sessions of task-specific training. The outpatient sample was evaluated for UE movement capabilities and monitored during a single session of task-specific training and the subsequent 22 hours outside clinical settings. MAIN OUTCOME MEASURES: Action Research Arm Test (ARAT) and acceleration metrics quantified from accelerometer recordings. RESULTS: Five metrics improved in the inpatient sample, along with UE function as measured on the ARAT: use ratio, magnitude ratio, variation ratio, median paretic UE acceleration magnitude, and paretic UE acceleration variability. Metric values were greater during task-specific training than in the free-living environment, and each metric was strongly associated with ARAT score. CONCLUSIONS: Multiple metrics that characterize different aspects of UE movement are responsive to change in function. Metric values are different during training than in the free-living environment, providing further evidence that what the paretic UE does in the clinic may not generalize to what it does in everyday life.

Visual regulation of overarm throwing performance.

This investigation examined whether visual feedback is used to make online adjustments during overarm throwing performance. Eight healthy, college-aged males able to throw in excess of 31.3 m/s with previous baseball pitching experience participated. Subjects performed maximal-effort overarm throws under a pretest condition and three test conditions. Under the randomly presented test conditions, the target either maintained its initial location or translated left or right of this location upon stride-foot contact. Subjects were instructed to project the ball to the terminal target location while maintaining maximal speed. Ball landing location, ball speed at release, and several kinematic parameters associated with the throwing motion were compared between conditions. The ball's global landing location within the horizontal dimension in the test-left and test-right conditions was more negative and positive, respectively, than in the pretest and test condition where the target maintained its initial position. Ball speed was lower in all test conditions relative to the pretest condition. Subjects also exhibited less lateral trunk tilt and greater peak pelvis linear velocity in all test conditions relative to the pretest. The overall time from stride-foot contact and ball release was not different between conditions, and no positional kinematic differences were observed between test conditions. The results of this study suggest that visually driven corrections occur late in the throwing motion without changes in the overall movement time. However, there do not appear to be specific features of the throwing motion that all subjects manipulate while making these adjustments.

Acute intermittent hypoxia boosts spinal plasticity in humans with tetraplegia.

Paired corticospinal-motoneuronal stimulation (PCMS) elicits spinal synaptic plasticity in humans with chronic incomplete cervical spinal cord injury (SCI). Here, we examined whether PCMS-induced plasticity could be potentiated by acute intermittent hypoxia (AIH), a treatment also known to induce spinal synaptic plasticity in humans with chronic incomplete cervical SCI. During PCMS, we used 180 pairs of stimuli where corticospinal volleys evoked by transcranial magnetic stimulation over the hand representation of the primary motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the first dorsal interosseous (FDI) muscle ~1-2 ms before the arrival of antidromic potentials elicited in motoneurons by electrical stimulation of the ulnar nerve. During AIH, participants were exposed to brief alternating episodes of hypoxic inspired gas (1 min episodes of 9% O2) and room air (1 min episodes of 20.9% O2). We examined corticospinal function by measuring motor evoked potentials (MEPs) elicited by cortical and subcortical stimulation of corticospinal axons and voluntary motor output in the FDI muscle before and after 30 min of PCMS combined with AIH (PCMS+AIH) or sham AIH (PCMS+sham-AIH). The amplitude of MEPs evoked by magnetic and electrical stimulation increased after both protocols, but most after PCMS+AIH, consistent with the hypothesis that their combined effects arise from spinal plasticity. Both protocols increased electromyographic activity in the FDI muscle to a similar extent. Thus, PCMS effects on spinal synapses of hand motoneurons can be potentiated by AIH. The possibility of different thresholds for physiological vs behavioral gains needs to be considered during combinatorial treatments.

Hindlimb motor responses evoked by microstimulation of the lumbar dorsal root ganglia during quiet standing.

Somatosensory afferent pathways have been a target for neural prostheses that seek to restore sensory feedback from amputated limbs and to recruit muscles paralyzed by neurological injury. These pathways supply inputs to spinal reflex circuits that are necessary for coordinating muscle activity in the lower limb. The dorsal root ganglia (DRG) is a potential site for accessing sensory neurons because DRG microstimulation selectively recruits major nerve branches of the cat hindlimb. Previous DRG microstimulation experiments have been performed in anesthetized animals, but effects on muscle recruitment and behavior in awake animals have not been examined. OBJECTIVE: The objective of the current study was to measure the effects of DRG microstimulation on evoking changes in hindlimb muscle activity during quiet standing. APPROACH: In this study, 32-channel penetrating microelectrode arrays were implanted chronically in the left L6 and L7 DRG of four cats. During each week of testing, one DRG electrode was selected to deliver microstimulation pulse-trains during quiet standing. Electromyographic (EMG) signals were recorded from intramuscular electrodes in ten hindlimb muscles, and ground-reaction forces (GRF) were measured under the foot of the implanted limb. MAIN RESULTS: DRG Microstimulation evoked a mix of excitatory and inhibitory responses across muscles. Response rates were highest when microstimulation was applied on the L7 array, producing more excitatory than inhibitory responses. Response rates for the L6 array were lower, and the composition of responses was more evenly balanced between excitation and inhibition. On approximately one third of testing weeks, microstimulation induced a transient unloading of the hindlimb as indicated by a decrease in GRF. Reciprocal inhibition at the knee was a prevalent response pattern across testing days which contributed to the unloading force on this subset of testing weeks. SIGNIFICANCE: Results show that single-channel microstimulation in the lumbar DRG evokes stereotyped patterns of muscle recruitment in awake animals, demonstrating that even limited sensory input can elicit hindlimb behavior. These findings imply that DRG microstimulation may have utility in neural prosthetic applications aimed at restoring somatosensory feedback and promoting motor function after neurological injury.

DRG microstimulation evokes postural responses in awake, standing felines.

OBJECTIVE: We have demonstrated previously that microstimulation in the dorsal root ganglia (DRG) can selectively evoke activity in primary afferent neurons in anesthetized cats. This study describes the results of experiments focused on characterizing the postural effects of DRG microstimulation in awake cats during quiet standing. APPROACH: To understand the parameters of stimulation that can affect these postural shifts, we measured changes in ground reaction forces (GRF) while varying stimulation location and amplitude. Four animals were chronically implanted at the L6 and L7 DRG with penetrating multichannel microelectrode arrays. During each week of testing, we identified electrode channels that recruited primary afferent neurons with fast (80-120 m s-1) and medium (30-75 m s-1) conduction velocities, and selected one channel to deliver current-controlled biphasic stimulation trains during quiet standing. MAIN RESULTS: Postural responses were identified by changes in GRFs and were characterized based on their magnitude and latency. During DRG microstimulation, animals did not exhibit obvious signs of distress or discomfort, which could be indicative of pain or aversion to a noxious sensation. Across 56 total weeks, 13 electrode channels evoked behavioral responses, as detected by a significant change in GRF. Stimulation amplitude modulated the magnitude of the GRF responses for these 13 channels (p  < 0.001). It was not possible to predict whether or not an electrode would drive a behavioral response based on information including conduction velocity, recruitment threshold, or the DRG in which it resided. SIGNIFICANCE: The distinct and repeatable effects on the postural response to low amplitude (<40 µA) DRG microstimulation support that this technique may be an effective way to restore somatosensory feedback after neurological injuries such as amputation.

Potentiating paired corticospinal-motoneuronal plasticity after spinal cord injury.

BACKGROUND: Paired corticospinal-motoneuronal stimulation (PCMS) increases corticospinal transmission in humans with chronic incomplete spinal cord injury (SCI). OBJECTIVE/HYPOTHESIS: Here, we examine whether increases in the excitability of spinal motoneurons, by performing voluntary activity, could potentiate PCMS effects on corticospinal transmission. METHODS: During PCMS, we used 100 pairs of stimuli where corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the hand representation of the primary motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the first dorsal interosseous (FDI) muscle ∼1-2 ms before antidromic potentials were elicited in motoneurons by electrical stimulation of the ulnar nerve. PCMS was applied at rest (PCMSrest) and during a small level of isometric index finger abduction (PCMSactive) on separate days. Motor evoked potentials (MEPs) elicited by TMS and electrical stimulation were measured in the FDI muscle before and after each protocol in humans with and without (controls) chronic cervical SCI. RESULTS: We found in control participants that MEPs elicited by TMS and electrical stimulation increased to a similar extent after both PCMS protocols for ∼30 min. Whereas, in humans with SCI, MEPs elicited by TMS and electrical stimulation increased to a larger extent after PCMSactive compared with PCMSrest. Importantly, SCI participants who did not respond to PCMSrest responded after PCMSactive and those who responded to both protocols showed larger increments in corticospinal transmission after PCMSactive. CONCLUSIONS: Our findings suggest that muscle contraction during PCMS potentiates corticospinal transmission. PCMS applied during voluntary activity may represent a strategy to boost spinal plasticity after SCI.

Motor Sequence Learning Is Associated With Hippocampal Subfield Volume in Humans With Medial Temporal Lobe Epilepsy.

Objectives: Medial temporal lobe epilepsy (mTLE) is characterized by decreased hippocampal volume, which results in motor memory consolidation impairments. However, the extent to which motor memory acquisition are affected in humans with mTLE remains poorly understood. We therefore examined the extent to which learning of a motor tapping sequence task is affected by mTLE. Methods: MRI volumetric analysis was performed using a T1-weighted three-dimensional gradient echo sequence in 15 patients with right mTLE and 15 control subjects. Subjects trained on a motor sequence tapping task with the left hand in right mTLE and non-dominant hand in neurologically-intact controls. Results: The number of correct sequences performed by the mTLE patient group increased after training, albeit to a lesser extent than the control group. Although hippocampal subfield volume was reduced in mTLE relative to controls, no differences were observed in the volumes of other brain areas including thalamus, caudate, putamen and amygdala. Correlations between hippocampal subfield volumes and the change in pre- to post-training performance indicated that the volume of hippocampal subfield CA2-3 was associated with motor sequence learning in patients with mTLE. Significance: These results provide evidence that individuals with mTLE exhibit learning on a motor sequence task. Learning is linked to the volume of hippocampal subfield CA2-3, supporting a role of the hippocampus in motor memory acquisition. Highlights -Humans with mTLE exhibit learning on a motor tapping sequence task but not to the same extent as neurologically-intact controls.-Hippocampal subfield volumes are significantly reduced after mTLE. Surrounding brain area volumes do not show abnormalities.-Hippocampal subfield CA2-3 volume is associated with motor sequence learning in humans with mTLE.

Acute intermittent hypoxia enhances corticospinal synaptic plasticity in humans.

Acute intermittent hypoxia (AIH) enhances voluntary motor output in humans with central nervous system damage. The neural mechanisms contributing to these beneficial effects are unknown. We examined corticospinal function by evaluating motor evoked potentials (MEPs) elicited by cortical and subcortical stimulation of corticospinal axons and the activity in intracortical circuits in a finger muscle before and after 30 min of AIH or sham AIH. We found that the amplitude of cortically and subcortically elicited MEPs increased for 75 min after AIH but not sham AIH while intracortical activity remained unchanged. To examine further these subcortical effects, we assessed spike-timing dependent plasticity (STDP) targeting spinal synapses and the excitability of spinal motoneurons. Notably, AIH increased STDP outcomes while spinal motoneuron excitability remained unchanged. Our results provide the first evidence that AIH changes corticospinal function in humans, likely by altering corticospinal-motoneuronal synaptic transmission. AIH may represent a novel noninvasive approach for inducing spinal plasticity in humans.

Distinct Interactions between Fronto-Parietal and Default Mode Networks in Impaired Consciousness.

Existing evidence suggests that the default-mode network (DMN) and fronto-pariatal network (FPN) play an important role in altered states of consciousness. However, the brain mechanisms underlying impaired consciousness and the specific network interactions involved are not well understood. We studied the topological properties of brain functional networks using resting-state functional MRI data acquired from 18 patients (11 vegetative state/unresponsive wakefulness syndrome, VS/UWS, and 7 minimally conscious state, MCS) and compared these properties with those of healthy controls. We identified that the topological properties in DMN and FPN are anti-correlated which comes, in part, from the contribution of interactions between dorsolateral prefrontal cortex of the FPN and precuneus of the DMN. Notably, altered nodal connectivity strength was distance-dependent, with most disruptions appearing in long-distance connections within the FPN but in short-distance connections within the DMN. A multivariate pattern-classification analysis revealed that combination of topological patterns between the FPN and DMN could predict conscious state more effectively than connectivity within either network. Taken together, our results imply distinct interactions between the FPN and DMN, which may mediate conscious state.

High-Intensity, Unilateral Resistance Training of a Non-Paretic Muscle Group Increases Active Range of Motion in a Severely Paretic Upper Extremity Muscle Group after Stroke.

Limited rehabilitation strategies are available for movement restoration when paresis is too severe following stroke. Previous research has shown that high-intensity resistance training of one muscle group enhances strength of the homologous, contralateral muscle group in neurologically intact adults. How this "cross education" phenomenon might be exploited to moderate severe weakness in an upper extremity muscle group after stroke is not well understood. The primary aim of this study was to examine adaptations in force-generating capacity of severely paretic wrist extensors resulting from high intensity, dynamic contractions of the non-paretic wrist extensors. A secondary, exploratory aim was to probe neural adaptations in a subset of participants from each sample using a single-pulse, transcranial magnetic stimulation (TMS) protocol. Separate samples of neurologically intact controls (n = 7) and individuals ≥4 months post stroke (n = 6) underwent 16 sessions of training. Following training, one-repetition maximum of the untrained wrist extensors in the control group and active range of motion of the untrained, paretic wrist extensors in the stroke group were significantly increased. No changes in corticospinal excitability, intracortical inhibition, or interhemispheric inhibition were observed in control participants. Both stroke participants who underwent TMS testing, however, exhibited increased voluntary muscle activation following the intervention. In addition, motor-evoked potentials that were unobtainable prior to the intervention were readily elicited afterwards in a stroke participant. Results of this study demonstrate that high-intensity resistance training of a non-paretic upper extremity muscle group can enhance voluntary muscle activation and force-generating capacity of a severely paretic muscle group after stroke. There is also preliminary evidence that corticospinal adaptations may accompany these gains.

Resting-state functional connectivity and its association with multiple domains of upper-extremity function in chronic stroke.

BACKGROUND: Recent work has shown that resting-state functional connectivity (rsFC) between homotopic, motor-related brain regions is associated with upper-extremity control early after stroke. OBJECTIVES: This study examined various patterns of rsFC in chronic stroke, a time at which extensive neural reorganization has occurred. Associations between homotopic somatomotor connectivity and clinical measures, representing separate domains of upper-extremity function, were determined. METHODS: A total of 19 persons ≥6 months poststroke participated. Four connectivity patterns within a somatomotor network were quantified using functional magnetic resonance imaging. Upper-extremity gross muscle activation, control, and real-world use were evaluated with the Motricity Index, Action Research Arm Test, and accelerometry, respectively. RESULTS: Connectivity between homotopic regions was stronger than that in the contralesional and ipsilesional hemispheres. No differences in connectivity strength were noted between homotopic pairs, indicating that a specific brain structure was not driving somatomotor network connectivity. Homotopic connectivity was significantly associated with both upper-extremity control (r = 0.53; P= .02) and real-world use (r = 0.54; P= .02); however, there was no association with gross muscle activation (r = 0.23; P=.34). The combination of clinical measures accounted for 40% of the variance in rsFC (= .05). CONCLUSIONS: The results reported here expand on previous findings, indicating that homotopic rsFC persists in chronic stroke and discriminates between varying levels of upper-extremity control and real-world use. Further work is needed to evaluate its adequacy as a biomarker of motor recovery following stroke.

Associations between timing in the baseball pitch and shoulder kinetics, elbow kinetics, and ball speed.

BACKGROUND: A baseball pitcher's ability to maximize ball speed while avoiding shoulder and elbow injuries is an important determinant of a successful career. Pitching injuries are attributed to microtrauma brought about by the repetitive stress of high-magnitude shoulder and elbow kinetics. HYPOTHESIS: Over a number of pitches, variations in timing peak angular velocities of trunk segment rotations will be significantly associated with ball speed and upper extremity kinetic parameters. STUDY DESIGN: Descriptive laboratory study. METHODS: Kinematic and kinetic data were derived from 9 to 15 fastball pitches performed by 16 active, healthy collegiate (n = 8) and professional (n = 8) pitchers via 3-dimensional motion capture (240 Hz). Each pitch was decomposed into 4 phases corresponding to the time between peak angular velocities of sequential body segment rotations. Four mixed models were used to evaluate which phases varied significantly in relation to ball speed, peak shoulder proximal force, peak shoulder internal rotation torque, and peak elbow varus torque. Mixed-model parameter coefficient estimates were used to quantify the influence of these variations in timing on ball speed and upper extremity kinetics. RESULTS: All 4 mixed models were significant (P < .05). The time from stride-foot contact to peak pelvis angular velocity varied significantly in relation to all upper extremity kinetic parameters and ball speed. Increased time in this phase correlated with decreases in all parameters. Decreased ball speed also correlated with increased time between peak upper torso and elbow extension angular velocities. Decreased shoulder proximal force also correlated with increased time between peak pelvis and upper torso angular velocities. CONCLUSION: There are specific phases that vary in relation to ball speed and upper extremity kinetic parameters, reinforcing the importance of effectively and consistently timing segmental interactions. For the specific interactions that varied significantly, increased phase times were associated with decreased kinetics and ball speed. CLINICAL RELEVANCE: Although increased time within specific phases correlates with decreases in the magnitude of upper extremity kinetics linked to overuse injuries, it also correlates with decreased ball speed. Based on these findings, it may appear that minimizing the risk of injury (ie, decreased kinetics) and maximizing performance quality (ie, increased ball speed) are incompatible with one another. However, there may be an optimal balance in timing that is effective for satisfying both outcomes.

Sensorimotor control in overarm throwing.

Goal-directed movement is possible because the cortical regions regulating movement have continuous access to visual information. Extensive research from the various domains of motor control (i.e., neurophysiology, neuropsychology, and psychophysics) has documented the extent to which the unremitting availability of visual information enables the sensorimotor system to facilitate online control of goal-directed limb movement. However, the control mechanism guiding appreciably more complex movements characterized by ballistic, whole-body coordination is not well understood. In the overarm throw, for example, joint rotations must be optimally timed between body segments to exploit the passive flow of kinetic energy and, in turn, maximize projectile speed while maintaining accuracy. The purpose of this review is to draw from the various research domains in motor control and speculate on the nature of the sensorimotor control mechanism facilitating overarm throwing performance.

Examining impulse-variability in overarm throwing.

The purpose of this study was to examine variability in overarm throwing velocity and spatial output error at various percentages of maximum to test the prediction of an inverted-U function as predicted by impulse-variability theory and a speed-accuracy trade-off as predicted by Fitts' Law Thirty subjects (16 skilled, 14 unskilled) were instructed to throw a tennis ball at seven percentages of their maximum velocity (40-100%) in random order (9 trials per condition) at a target 30 feet away. Throwing velocity was measured with a radar gun and interpreted as an index of overall systemic power output. Within-subject throwing velocity variability was examined using within-subjects repeated-measures ANOVAs (7 repeated conditions) with built-in polynomial contrasts. Spatial error was analyzed using mixed model regression. Results indicated a quadratic fit with variability in throwing velocity increasing from 40% up to 60%, where it peaked, and then decreasing at each subsequent interval to maximum (p < .001, η2 = .555). There was no linear relationship between speed and accuracy. Overall, these data support the notion of an inverted-U function in overarm throwing velocity variability as both skilled and unskilled subjects approach maximum effort. However, these data do not support the notion of a speed-accuracy trade-off. The consistent demonstration of an inverted-U function associated with systemic power output variability indicates an enhanced capability to regulate aspects of force production and relative timing between segments as individuals approach maximum effort, even in a complex ballistic skill.

Impulse-variability theory: implications for ballistic, multijoint motor skill performance.

Impulse-variability theory (R. A. Schmidt, H. N. Zelaznik, B. Hawkins, J. S. Frank, & J. T. Quinn, 1979) accounts for the curvilinear relationship between the magnitude and resulting variability of the muscular forces that influence the success of goal-directed limb movements. The historical roots of impulse-variability theory are reviewed in the 1st part of this article, including the relationship between movement speed and spatial error. The authors then address the relevance of impulse-variability theory for the control of ballistic, multijoint skills, such as throwing, striking, and kicking. These types of skills provide a stark contrast to the relatively simple, minimal degrees of freedom movements that characterized early research. However, the inherent demand for ballistic force generation is a strong parallel between these simple laboratory tasks and multijoint motor skills. Therefore, the authors conclude by recommending experimental procedures for evaluating the adequacy of impulse variability as a theoretical model within the context of ballistic, multijoint motor skill performance.